(Not Applicable)
Trace chemical detection of explosives, i.e., the art of detecting explosive materials from minute quantities of vapor and/or microscopic particles (hereinafter referred to as xe2x80x98particlesxe2x80x99), can be an important aspect of many physical security systems. Among the challenges currently confronting researchers in this area is the problem of how to collect the explosive sample and transport it to the detector without major losses. In many applications, especially applications involving the general public such as airport passenger screening, swipe collection of particles via direct physical contact with the person or object to be screened for explosives is either too physically invasive or time consuming, so it is necessary to base the collection process on air flows. But the vapor and/or airborne particle material that is collected in such air flows is usually far more dilute than the detector is capable of measuring, and the air flow is often too large to be directly accommodated by the detector. These disparities give rise to preconcentrators, devices which take a trace sample of a material from a large incoming air flow and concentrate the material into a smaller volume before it is introduced into a trace detector.
U.S. Pat. No. 5,854,431 of Linker et al discloses a single stage preconcentrator 10 for use in collecting particles from an air stream that passes over a person or object under observation. Preconcentrator 10 includes a screen 14 disposed between input and output air stream valves 22 and 22xe2x80x2, respectively, which valves are secured together to form a layered arrangement whereby gas to be tested passes through open valve 22, through screen 14 (where particles are deposited), and exits through open valve 22xe2x80x2. A fan or other gas moving device may be situated downstream of output valve 22xe2x80x2, and the source upstream of input valve 22 may be the output of a booth through which people or objects being tested may pass.
As set forth in the ""431 patent, screen 14 is preferably formed of a metallic felt made from very thin metal filaments, with diameters ranging from 1 to 80 micron. As a comparison, human hair has a diameter between 70 and 100 micron. The felt is a pleatable and weldable stainless steel matrix, produced by the sintering of a composite metal fiber. The preferred material is Bekipor(copyright) ST, produced in North Carolina by Bekaert Fibre Technologies of Belgium. The preferred configuration of screen 14 was pleated, with the folds being parallel to the flow of gas during desorption. A particular advantage of this screen material is that it may be resistively heated to release the particles from the surface by applying an electric potential across its surface.
Single stage preconcentrator 10 of the ""431 patent operates as follows: firstxe2x80x94both valves 22, 22xe2x80x2 are open and air to be tested flows through screen 14, which is not being heated and which absorbs particles from the air to be tested; secondxe2x80x94both valves 22, 22xe2x80x2 are closed and screen 14 is heated to desorb collected particles; and thirdxe2x80x94a carrier gas 34 (air or inert gas) is provided to move desorbed particles in a direction parallel to the pleated surface of screen 14 to a detector at output 40.
A problem with this device is that the output 40 flows at about 4 liters/minute in order to move the particles from screen 14, while the detector prefers an input on the order of 0.1 to 0.5 liters/minute.
Simple flow restrictors would not work in this application, because the particles may adsorb to the restrictor instead of proceeding to the detector. In addition, any solution to this problem must be capable of being reset after every test so that the results of the previous test do not effect the next test.
It is an object of this invention to provide a 2-stage preconcentrator to enable particles to be moved to a detector under conditions acceptable to the detector, and to provide for cleaning of one stage while the other stage is operating.
To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied and broadly described herein, the present invention may comprise a device for concentrating particles in a high volume gas flow for detection in a low volume gas comprising a first preconcentrator comprising a first structure for retaining particles in a first gas flow path through which a first gas flows at a relatively high flow; means for selectively stopping the first gas flow; and a second gas flow path through which gas flows at an intermediate flow for moving the particles from the first structure to a second preconcentrator coupled to said first preconcentrator. The second preconcentrator comprises a second structure for retaining particles in the second gas flow path; means for selectively stopping the second gas flow; and a third gas flow path through which gas flows at a relatively low rate for moving the particles from the second structure to a detector.
Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.